The present invention relates to a dc SQUID to be used for measurement of a micromagnetic field such as instrumentation of biomagnetism, magnetic airborne detecting or the like.
There is known a Josephson junction of the quasi-planar type in which an upper electrode is formed on a lower electrode through a barrier layer, and the surface of the lower electrode is weak-linked to the surface of the upper electrode by a bridge.
In such a Josephson junction of the quasi-planar type, the weak-link length is determined by the thickness of the barrier layer interposed between the upper electrode and the lower electrode. Thus, the Josephson junction of the quasi-planar type is advantageous in that the weak-link length is determined by the adjustment of te film thickness of which control is relatively easy, as compared with a Josephson junction of the planar type requiring micro-machinings on the planar face.
FIG. 7 shows an example of the arrangement of a dc SQUID element having a conventional Josephson junction of the quasi-planar type with interlaminar insulation and barrier layers seen in a perspective manner. FIG. 7 (a) is a general plan view, while FIG. 7 (b) is an enlarged view of a portion B in FIG. 7 (a).
In this example, the dc SQUID has an arrangement of total four superconducting thin films in lamination in which a modulation coil 1 and an input coil 2 are formed at the lowermost layer on a substrate, and an input coil leading electrode 21 and a groundplane 3 are formed at the layer above the lowermost layer. A SQUID ring 4 is formed at the layer above the layer of the electrode 21 and the groundplane 3, and a counter electrode 5 is formed at the uppermost layer.
The SQUID ring 4 and the counter electrode 5 are weak-linked at two portions to each other, through a barrier layer (not shown) interposed therebetween, by a bridge 6 striding over the counter electrode 5 at both lateral edges thereof. Thus, two Josephson junctions 71, 72 are formed.
The arrangement above-mentioned may be produced, for example, according to steps as shown in plan views of FIGS. 8 to 11, in which the interlaminar insulation and barrier layers are not shown as seen in a perspective manner.
First, a superconducting thin film of the Nb type or the like is deposited on the substrate, which is then patterned to obtain the modulation coil 1 and the input coil 2. This state is shown in FIG. 8.
After an interlaminar insulation layer and contact holes are formed, a superconducting thin film is deposited from above. This film is patterned to form the groundplane 3 and the input coil leading electrode 1. This state is shown in FIG. 9.
An interlaminar insulation layer and contact holes are then formed, and a superconducting thin film is deposited from above. This film is patterned to form the SQUID ring 4 as shown in FIG. 10.
A barrier layer is formed on the entire surface of the SQUID ring 4 serving as the lower electrode. A superconducting thin film is then deposited on this barrier layer. This film is patterned to form the counter electrode 5 also serving as the upper electrode. This state is shown in FIG. 11.
Finally, a superconducting thin film is deposited from above and the bridge 6 having the pattern as shown in FIG. 7 is formed.
In a tunnel-type Josephson junction element or the like, it is not possible to adjust the value of critical current after the element has been formed. Accordingly, the order of preparing the respective layers above-mentioned does not particularly cause trouble. In a quasi-planar-type Josephson junction element, however, the value of critical current can be adjusted while monitoring the same by anodic oxidation or the like. To utilize such advantage, the bridge 6 is preferably formed at the last step.
In this point of view, the conventional element arrangement and manufacturing method mentioned earlier are reasonable in that the SQUID ring 4 and the counter electrode 5 respectively serving as the lower and upper electrodes for forming Josephson junction portions, are formed on the modulation coil 1 and the input coil 2, and the bridge 6 is formed at the uppermost layer.
When a number of films are laminated in the manner above-mentioned, it is inevitable that the surface flatness and quality of each of films of layers at upper positions are deteriorated more.
In the quasi-planar-type Josephson junction element, the film flatness and quality of the lower electrode (the SQUID ring 4 in the example above-mentioned) exerts a great influence upon the quality of the barrier layer which is formed above the lower electrode and which not only assures the insulation with respect to the upper electrode (the counter electrode 5 in the example above-mentioned), but also determines the weak-link length. In view of the foregoing, if such advantage of the quasi-planar-type Josephson junction element as to adjust the value of critical current is sacrificed, it may be considered rather preferable to dispose the lower electrode and the upper electrode at layers as lower as possible in the element, and in an extreme case, to dispose the Josephson junction portions at the lowermost layer.